Prospecting



1951 w. M. STRATFORD 2,562,962

PROSPECTING Filed 951 2 Sheets-Sheet 1 FIG. 3.

REMOVABLE SHIELD COVE/P TORO/DAL DETECTOR I'NVENTOR- WILL/AM M.STRATFORD I PREAMPL/F/ER BY Q QL V, "I 9 M g. i H,

m/c/r SHELL 0F LEAD 0/? OTHER SU/TABLE MATERIAL OF H/GH DENSITYATTQR/VE'YK? 1951 w. M. STRATFORD 2,562,962

PROSPECTING Filed Feb. 7, 1.951 2 Sheets-Sheet 2 MAP OF STREAM DRA/NAGESYSTEM, 66 ILLUSTRAT/NG UPSTREAM PROSPECT/N6 67 TO LOCATE SOURCE OFANOMALOUS GAMMA X 68 RADIOACT/V/TK SAMPL/NG POINTS ARE i 90 X MARKED"Xf'F/GURES ADJACENT SAMPLING 00 POINTS ARE cou/vTs PER SECOND PER 66I00 GMS. 0F SAMPLE. v v 88 5 Y 0w FORK BUR/0 OREBOX a7 69 woe/40w 53 mm"F" a ram E 1 75 ROCKS //v THIS AREA AVERAGE 68.

' INVENTOR.

W/LL/AM M STRATFORD W W W W $2 EA! MM ATmAMe-z Patented Aug. 7, 1951UNITED STATES TENT OFFICE PROSPECTENG William M. Stratford, New York, N.Y, assignmto The Texas Company, New York, N. Y., a corporation ofDelaware 1 Claim. 1

This invention is concerned with prospecting for mineral deposits,particularly those of metallic ores, and provides improvements whichfacilitate the location of such deposits. This applicae tion is adivision of co-pending application Serial No. 13,845, filed jointlyMarch 9, 1948, by Charles F. Teichmann, Gerhard l-lerzog and myself.

Mineral prospecting is as old as mans use of metals, but despite itsantiquity and the successful application of geophysical methods in a fewinstances during recent years, it is still far more ofan art than ascience. Most of the large deposits of the base and precious metals owetheir discovery to chance observation rather than to scientific surveys.Consumption of metals increases and ore reserves decrease, thusincreasing the incentive for discovering new deposits.

Nevertheless, discovery has not kept pace with depletion, probablybecause the prospecting art has not kept pace with scientificdevelopment in other fields and is, in the majority of instances,inadequate for the location of ore bodies which do not disclosethemselves through surface manifestations such as outcrops, gossan andthe other ancient indicia employed by prospectors the World over. In afew special instances, such as pronounced magnetic, electrical orgravimetric anomalies, concealed ore bodies have been discovered bygeophysical methods, but many ore bodies are not accompanied by suchanomalies. Exploration by means of raises, cross cuts and other mineopenings and by core or churn drilling is expensive and far fromcertain, for valuable ore bodies frequently are missed by a matter of afew feet. In short, there is a distinct need for improvements in orefinding. The instant invention supplies such need, at least in part.

As disclosed in co-pending application Serial No. 13,842, filed March 9,1948, by Gerhard Herzog, it has been discovered that many ore bodies,which may or may not be radioactive themselves are accompanied bydetectable radioactive auras in the substantially barren country rocksinwhich they occur. By carrying an efficient gamma ray detector along atraverse in rock and they may appear at distances far in excess of therange of penetration of significant amountsof radiation originating inthe ore body itself. Gamma ray surveys conducted as described above,both underground and on the surface, have successfully located orebodies through as much as two hundred feet of barren country rock, whichconstitutes a barrier capable of absorbing any detectable amounts ofeven, the hardest gamma radiation.

It is disclosed in the parent application, Serial No. 13,845, filedMarch 9, 1948, that the auras described above are also detectable byanother method, which is frequently more convenient, since it need notinvolve the movement of gamma ray detectors in the field to determineradiation intensities emanated from the undisturbed rock. It is statedin the parent application that gamma ray anomalies indicative of thepresence'of ore deposits may be discovered by taking rock or soilsamples at a plurality of spaced points on or under the earths surface,measuring accurately the intensity of the radiation (particularly gammaradiation) emitted by the individual samples and determirdng theintensity of radiation per unit mass (Weight or volume) of theindividual samples. These latter values are then related to the geometryof the survey, for example by plotting them at the respective points ona map representative of the area in Which the samples are taken anddrawing contours through points of equal intensity to form so-calledisoradins, or by plotting the values as ordinates with the traversealong which they were taken as abscissa. Anomalous zones of high or lowgamma ray intensities may thus be revealed.

In general, Weight represents sample mass more accurately than volume,but the latter may be used when dealing with samples of approximatelythe same specific gravity and screen analysis.

I have discovered that gamma ray anomalies indicative of an ore depositlying within a stream drainage system may be discovered by determiningthe approximate average intensity of gamma rays per unit mass emitted bythe rocks in the area of the drainage system, taking a first earthdetritus sample at a starting point downstream in the drainage systemand determining the intensity of gamma rays per unit mass of saiddetritus sample. Thereafter, the determined intensity of the detritussample is compared with the average intensity determined for the rocksof the drainage system so as to find which intensity is higher. Earthdetritus samples are taken'from each branch of a fork in the drainagesystem upstream from the starting point to determine which of theseupstream samples emits the higher intensity of gamma radiation per unitmass. The

prospecting is continued up the branch whose sample has the higherrelative intensity of gamma radiation per unit mass when the intensityper unit mass of the detritus sample from the starting point is higherthan the average intensity per unit mass of the rocks, but proceeds upthe other branch when the intensity per unit mass of said detritussample is lower than the average intensity per unit mass of the rocks.

Although theoretically any type of gamma radiation detector might beemployed in the practice of the invention, provided that the observationtime was suihciently long to determine acc urately the intensity of thegamma radiation emitted by each sample, practical considerations requirethe use of a detector Of high eificiency for gamma rays-several timesthat of the conventional Geiger--Mueller counter. The samples are ofnecessity relatively small so that the source of radiation is minute. Inmany cases, small but significant differences in intensity cannot bedetected at all with a Geiger-Mueller counter and other detectors of thesame order of emciency, no matter how long is the period of observation,because the variations in background which occur from one observationperiod to another are greater than the diiierence in intensity to bedetermined. Even when the difference in gamma ray intensity betweensamples is relatively large, required observation times for each samplewith a Geiger-Mueller detector may be a matter of days and hence beyondall limits of practicality.

Fortunately, suitable gamma ray detectors which permit accuratedetermination of the difference of gamma ray intensities between smallsamples have been developed. One such detector is described and claimedin U. S. Patent No. 2,397,071, granted March 19, 1946. An even moredesirable detector is that described and claimed inU. S. Patent No.2,397,072. Both of these detectors are of the multiple plate cathodetype and consist essentially of a stack of perforated disks disposedcoaxially and spaced parallel to each other, with one or more anodeWires running through the perforations transverse to the disk surfaces.The large cathode area per unit of active volume thus obtained increasesthe emciency for gamma rays several times, without increasing efficiencyfor the detection of back= ground proportionately. Thus counters of thistype have a gamma ray counting efficiency of 2.5% or more as comparedwith an eihciency of about for the conventional Geiger-Mueller counter.

All known detectors capable of detecting gamma radiation are alsocapable of detecting other radiation, including alpha and beta rays andcosmic radiation. This other radiation thus detected constitutes thebackground against which the gamma radiation intensity must be measured.Alpha and beta rays have low penetrating power and the entry of theserays into the counter from outside sources may be prevented byappropriate shielding. However, alpha and beta rays originating in thedetector itself due to slight contamination of the materials of which thcounter is made cannot be eliminated and contribute to the background.Cosmic rays also contribute to the background. Their gamma raycomponents may be prevented from contributing substantially to thebackground by adequate shielding, say several inches of lead or otherhigh density metal, but th so-called penetratin particles of cosmicradiation are many times more penetrating than gamma rays and cannot bestopped with practical amounts of shielding, as witness their occurrenceseveral thousand feet underground. In short, it is possible to decrease,but not eliminate background by shielding.

The radiation which constitutes the background is emitted sporadicallyand at random. In the practice of the invention it is desirable toreduce th background as much as practicable in order to reduce errorarising from this fluctuation as well as to increase contrast betweenmeasured intensities. The approach to uniformity of background is alsofurthered by selccting a place where the background is naturally low andconducting cemparative tests while th detector remains at that placeprotected by a constant amount of shielding. Thus, durin a given survey,the detector should be kept at a point to which the samples are broughtand should be protected on top, bottom and sides by adequate shielding,say several inches of lead. If the survey is being conducted in a miningdistrict, an underground location in rock or" low radioactivity andselected for its low background is desirable, since in this Way theeffect of cosmic rays may be reduced.

The background should b checked frequently by observing the activity ofthe detector with no sam pl present, since it is necessary to subtractthe background value from the total observed radiation in order todetermine which part is due to the sample.

Since gamma ray detectors in general do not detect with anythingapproaching 106% efiiciency, the measurements of gamma rays which aremade in the practice of the invention are comparative rather thanabsolute, but this presents no obstacle if detection efliciencies aresubstantially uniform from sample to sample.

These and other aspects of the invention will be clearly understood inthe light of the following detailed description of presently preferredpractices, taken in conjunction with the accompanying drawings, inwhich:

Fig. l is an elevation, partly in section, of a preferred form ofradiation detector for the practice of the invention;

Fig. 2 is a horizontal section taken through the apparatus of Fig. 1along the line 2-2;

Fig. 3 is a diagram illustrating the shielding of the detector of Figs.1 and 2; and

Fig. a is a map illustrating the systematic sampling of a drainagesystem undertaken to discover an upstream mineral deposit.

A toroidal or cup-shaped detector constructed in accordance with U. S.Patent No. 2,397,072, granted March 19, 1946, and particularly adaptedto the practice of the instant invention is illustrated by Figs. 1 and2. It comprises a spaced stack of annular silver cathode plates l5disposed in an annular envelope II and electrically connected togetherin parallel. Conveniently, at least the outer wall of the envelope ismetallic and the cathodes are electrically connected thereto. Eachcathode plate has a series of symmetrically disposed holes l2, the holesin the several plates being in alignment to permit the passage throughthe plates of a plurality of tungsten anode Wires [3. These wires areparallel to each other and perpendicular to the plates and are disposedrespectively on the axes of the several rows of holes. The wires arestretched taut between insulators l4, l5 at their ends and are connectedin parallel with each other to a common conductor H5 in plate form. Alead from this plate passes through an insulator bushing ll and thenceto a conventional counter circuit (not shown) including a D. C. highvoltage supply, a pre-amplifier, an amplifier, a scaling circuit, and arecorder. Each cathode plate is provided with a notch to aid inalignment of the holes in the plates and a rib I8 fastened to theoutside wall of the envelope passes through the several notches andprevents the plates from turning within the envelope. Spacers l9 aredisposed between the plates immediately inside the envelope to hold themapart in fixed relationship with each other, say on 3% inch centers.

The top of the annular envelope is closed by an annular plate 28extending from the inside Wall of the envelope to the outside wall. Theinside wall 2lA of the envelope defines the side of a deep cylindricalcup 2| in which a sample to be investigated is placed. The bottom of thecup is defined by a plate 22. The bottom of the detector as a whole isclosed by a cylindrical plate 23 through which the bushing ll passes.The outside wall of the envelope may be screwed onto a container holdinga pre-amplifier (see Fig. 2).

The entire envelope is gas tight and is filled with a suitableatmosphere, say a mixture of alcohol and argon.

In operation, a high potential difference is established between theanodes and the cathodes, the potential difference being nearly, butnotquite, high enough to cause a discharge to take place. If an ionizingray passes into the detector, a discharge may take place with resultantcurrent fiow which produces a count. The discharge ceases after a shortperiod of time, after which the counter is again in condition to countionizing rays.

The toroidal detector is particularly desirable for the practice of theinvention since a ray orig inating in a sample placed in its cup is muchmore likely to enter the active volume of the detector and be detectedthan if the sample were placed outside the counter. In short, the sampleis substantially surrounded by active detector volume and hence theregistered intensity of its radiation tends to be increased.

In the practice of the invention it is important that the background bemaintained as low as possible and be uniform from sample to sample in agiven survey. In order to bring about this diminution in background andto maintain it uniform, a site should first be chosen at which thebackground is low. Rock tends to stop cosmic rays and reduce theintensity of background from this source. Accordingly, especially insurveys in known mining districts, it may be convenient to place thedetect-or underground, for example, in an abandoned stope, drift orcrosscut. In choosing the location, a preliminary survey should be madewith a detector having a high efiiciency for the gamma rays emanatedfrom the ground. When an underground space, say a stop-e, is found inwhich the background intensity is at a minimum, the toroidal detector isset up as shown in Fig. 3. Thus the toroidal detector is mounted abovethe preamplifier and both are disposed in a lead shield having a thickbottom, thick walls, and a removable cover, likewise lead. Generallyspeaking, the thicker the 6 feet, above the detector in order to reducethe effect of cosmic radiation as much as possible.

Individual samples, obtained in a manner hereinafter described, are keptseparate and each is crushed to approximately the same maximum size andsize distribution. Crushing to minus 10 mesh is desirable and even finercrushing may give better results. After the samples have been crushed arepresentative portion of each is placed in the counter. A thin walledglass test tube may be employed to hold the sample and the test tubedropped into the cup of the toroidal detector.

Samples of grams have been used with good results, although generallyspeaking, the bigger the sample the better. In any case, each sampletested in the detector should have approximately the same mass.Conveniently, the same volume of sample is taken each time and theweight of each sample is taken.

In carrying out the measurements of gamma radiation intensity inaccordance with the invention, it is essential to know the backgroundcount. Consequently, with the detector empty the background is measuredto discover its intensity, which varies sporadically within limits andmay also vary with the time of day. This latter variation is known asthe diurnal variation and should be determined by taking measurements ofthe background intensity at intervals during the day. The backgroundcount should be checked frequently, at least once a day.

After the background has been established, each sample in turn is placedin the detector and left there until a fixed number of counts has beenrecorded, the time for this standard count being accurately determinedin each case. When using a detector equipped with a scale of 16 sealercircuit, good results are obtained by tak ing 3200 counts for eachsample. The procedure of taking a constant number of counts elimihatesvariations in statistical error. The appro priate background count forthe duration of the observation and for the particular time of day isthen subtracted in each case. This back-- ground, as already indicated,varies with the time of day. The corrected count, i. e. the total countminus the background is now divided by the observation time to give avalue for gamma ray intensity, i. e. counts per unit time. This resultis in turn divided by the weight or volume of the sample, preferably theweight, to give a figure for intensity per unit mass.

Fig. 4 illustrates systematic sampling of a drainage system proceedingupstream to locate sources of anomalous gamma ray intensities found inthe sands of a stream bed or in suspended solids carried by the streamsome distance below a buried ore body. In the process illustrated byFig. l, sampling is begun at a point A down stream in the drainagebasin. The rocks in this basin show an average gamma ray intensity of 68counts per minute per 100 gms. At the first sample point downstream thesample gives an intensity of 75 per minute per 100 gms. Proceedingupstream a value of 76 is noted. Since both of these are above theaverage for the rock in the area, there is an indication that materialof higher gamma ray intensity has been carried into the stream fromabove, possibly by erosion of the radioactive aura from an ore bodywhich is itself not yet exposed by erosion. When the first fork B in thestream is attained, samples are taken on both branches. The intensity ofthe sample in the left hand branch is low, indicating that the materialof high gamma intensity is transported by the right hand branch in whichan intensity ratio of 76 is noted. Consequently the right hand branch isfollowed. At the next fork C a similar difference in intensity ratios isnoted from the samples of the respec-- tive branches and that giving thehighest intensity is followed, this procedure continuing upstream towardthe head waters past forks D, E, F and G. Finally, as the selectedbranch H is followed upstream an intensity of 90 counts per minute per100 gms. is noted, but further progress upstream gives lower intensitiesindicating that the material of high intensity is being washed into thestream below these points of low intensity. The stream bed is now leftand samples are taken uphill along the course that float from anout-cropping radioactive aura would necessarily take. Eventually, anarea is reached in which a closed isoradin of say 120 counts per minuteper 100 gms. can be plotted, with lower intensities away from thisisoradin and higher intensities within it. This is a posi' tive anomalywhich may be indicative of an ore body under the surface in theneighborhood of the closed isoradin. The positive anomaly sought havingbeen discovered, a detailed examination similar to that illustrated inFig. 6 of the co-pending application Serial No. 13,845, filed March 9,1948, can be carried out, this to be followed by drilling or sinking ifthe results seem to justify such a step.

In the investigation of a drainage system, the location of both positiveanomalies and negative anomalies may be sought by sampling upstream. Ifa downstream sample shows a low intensity of gamma rays per unit mass ascompared with the average intensity per unit mass of gamma rays emittedby the rocks of the drainage basin, the procedure is the same as thatdescribed in the case of Fig. 4, except that the direction of lowintensity, as indicated by successive sampling at forks, is pursuedupstream.

The treatment of wet samples obtained in prospecting with the method ofthe invention, for example, along a stream course, may be conducted invarious ways. The simplest fashion is to take samples of the sand in thestream bottom or a sample of the water itself bearing suspended solids.These samples are then evaporated to dryness and the procedurethereafter is as described for solid rock samples, except that usuallyno crushing is necessary. In evaporation of the samples to dryness,however, sources of alpha radiation usually are lost, so that if thedetermination of alpha radiation intensity is important the radioactivegases (say radon, actonon or thoron contained in the wet samples) shouldbe extracted by vacuum and subjected to detection prior to drying thesample.

In desert areas or on steep terrain on which water flow is irregular,deposits may be traced to their source through the practice of theinvention by conducting a sampling program uphill along the course ofthe float from an outcrop of a radioactive anomaly in the country rock.

Percolating ground waters may also carry dissolved or suspendedradioactive material from a mineral deposit or from its aura and it iswith in the contemplation of the invention to sample the ground water ofan area being prospected by laying out a grid in the area and samplingthe ground water systematically in bore holes at the grid intersectionsor at other known locations.

I claim:

In prospecting a stream drainage system to locate therein a gamma rayanomaly associated with a mineral deposit to be found, the improvementwhich comprises determining the approximate average intensity of gammarays per unit mass emitted by the rocks in the area of the drainagesystem, taking a first earth detritus sample at a starting pointdownstream in the drainage system and determining the intensity of gammarays per unit mass of said detritus sample, comparing the determinedintensity of said detritus sample with the average intensity determinedfor the rocks or" the drainage system so as to find which intensity ishigher, taking earth detritus samples from each branch of a fork in thedrainage system upstream from the starting point and determining whichof these upstream samples emits the higher intensity of gamma radiationper unit mass, proceeding up the branch whose sample has the higherrelative intensity per unit mass when the intensity per unit mass of thedetritus sample from the starting point is higher than the average intensity per unit mass of the rocks, but proceeding up the other branch whenthe intensity per unit mass of said detritus sample is lower than theaverage intensity per unit mass of the rocks.

VV'ILLIAM M. STRATFORD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Hall, Oct. 1940, pp. 870875.

Evans et al.: Review of Scientific Instruments, vol. 10, Nov. 1939, pp.332-336.

Locher and Weatherwax: Radiology, vol. 27, 1936, pp. 149457.

